1. Field of the Invention
The present invention relates to an imaging system that is suitably used in a medical diagnosis to take a still image, such as in general radiography, or to take a moving image, such as in fluoroscopy.
2. Description of the Related Art
In recent years, a radiation imaging apparatus using a flat panel detector (hereinafter simply referred to as “detector”) that is made of a semiconductor material has come into practical use as an imaging apparatus used in a medical image diagnosis or a nondestructive inspection by radiation. For example, in the medical image diagnosis, such radiation imaging apparatus is used as a digital imaging apparatus to take a still image, such as in general radiography, or to take a moving image, such as in fluoroscopy. As the detector, there is known an indirect-conversion detector obtained by combining a scintillator configured to convert radiation into light with a wavelength band that is detectable by a photoelectric conversion element, and a solid-state imaging element as a sensor for detecting the converted visible light. As the imaging apparatus, for example, for mammography and chest radiography, an imaging apparatus for taking a large-area still image, which uses amorphous silicon (a-Si) of 43 centimeter square at maximum, has been put into practical use.
In this case, the radiation imaging apparatus are desired to achieve technical objects such as high sensitivity, high-speed reading technology, increase in size, and cost reduction. However, amorphous silicon has insufficient semiconductor performance, which makes it difficult to achieve the demand particularly concerning high sensitivity and high-speed reading. In order to cover the shortcomings of the imaging element using amorphous silicon, a configuration including tiled large-area CMOS imaging elements has been put into practical use in recent years.
However, in a related-art amplification-type imaging element such as a CMOS imaging element, radiation may transmit through the scintillator to be exposed to the solid-state imaging element. In this case, there arises a problem in that a noise signal caused by direct incident radiation is superimposed on an image signal generated by the visible light. The noise signal caused by radiation that has directly entered the solid-state imaging element is called blinker noise.
Japanese Patent No. 3894534 discloses a radiation imaging apparatus including a radiation generator and a radiation sensor for converting, into an electrical signal, radiation that has been emitted from the radiation generator toward an object and has transmitted through the object. A signal value of each pixel of the radiation sensor is read twice in a radiation exposure time period. The blinker noise component is extracted as follows. A difference between a first signal of each pixel that is read through first reading in the radiation exposure time period and a second signal of each pixel that is read through second reading performed after the first reading in the radiation exposure time period is calculated, and thus an object component is removed. Then, the first signal is added to the second signal, and an absolute value of the noise component is subtracted from the added value to remove the noise component. In Japanese Patent No. 3894534, the difference between the first signal and the second signal that are read twice in the radiation exposure time period is calculated to remove the object component of the image, and thus the component of blinker noise is extracted. However, in the method of Japanese Patent No. 3894534, unless exactly the same amount of radiation is exposed when reading the first signal and the second signal that are read twice in the radiation exposure time period, when the difference between the first signal and the second signal is calculated, the object component cannot be completely removed, which causes failure in extraction of the blinker noise component. In the actual case, radiation emitted from the radiation generator is not always constant in amount, and always randomly varies. Therefore, it is virtually impossible to control the amount of radiation exposed when reading the first signal and the amount of radiation exposed when reading the second signal to be exactly the same.